The present invention relates to an improvement in a mass spectrometer using a plasma ion source and, more particularly, relates to a mass spectrometer using a plasma ion source improved so as to be suitable for detecting an ultra-trace element present in a sample to be analyzed.
Lately a mass spetrometer using a plasma ion source for analyzing an ultra-trace element present in a sample has been under extensive development. A representative example of conventional mass spectrometers using a plasma ion source is disclosed in a publication, Spectrochimica Acta, Vol. 42B, No. 5 (1987), pp. 705-712.
FIG. 4 schematically shows the structure of the apparatus described in the above mentioned publication. Referring to the figure, reference numeral 10 denotes plasma of a gas having the composition of a sample, 20 denotes a sampler, 70 denotes a first aperture plate, 80 denotes a mass filter (mass spectrometer), 90 denotes a second aperture plate, 100 denotes an ion detector, 120 denotes a skimmer, 130 denotes an ion extraction electrode, 140 denotes an Einzel lens, 150 denotes an energy analyzer, and 151 denotes a center plate disposed on the central axis of the energy analyzer.
In the apparatus of the described arrangement, ions generated in the plasma 10 are extracted therefrom by an ion extraction system composed of the sampler 20, the skimmer 120, and the ion extraction electrode 130 and then passed through the Einzel lens 140 and the energy analyzer 150 to be admitted into the mass filter 80, where they are subjected to mass spectrometry.
The center plate 151 disposed within the energy analyzer 150 has also a function to stop photons generated in the plasma 10 from entering the ion detector 100 and brings about such an effect as to improve the S/N ratio of the output signal of the ion detector and, hence, to lower the detection limit of the detector.
In the above described conventional apparatus, however, three steps of vacuum chambers have had to be provided in the course from the plasma 10 to the mass filter 80 and the Einzel lens 140 and the energy analyzer 150 have had to be disposed midway through the course. Thus, the construction becomes complex and the number of required parts becomes great therefore, such difficulties arise that the manufacturing cost becomes high and the operation and maintenance service become difficult. Further, because of the structural complexity of the apparatus, the transport efficiency of ions from the plasma 10 to the mass filter 80 is held low and, since the above described function to prevent the photons from entering the ion detector 100 is not performed satisfactorily as yet, the S/N ratio of the detected signal is still low, and therefore, the detection limit cannot be made as sufficiently low as desired for analyzing the composition of a trace in the sample.